1. Field of the Invention
The present invention relates to the detection of contamination in or on the surface of an optical element, in the respective form of surface discontinuities and particulate matter. By "discontinuities" is meant molecular matter which wets or craters the surface to form a greater than point contact or surface irregularity. By "particulate matter" is meant essentially a point contact with the surface. the difference between the two is primarily one of detectability, because both deleteriously affect the optical quality of the element.
2. Description of Related Art and Background Considerations
High quality, precision optical equipment often cannot perform to its optimum capabilities when its surfaces are contaminated. For example, an increased requirement of LWIR (long wave infrared) Space Surveillance Sensors is to detect low-radiance targets in the presence of intensive out-of-field sources, such as the earth and sun. This requirement places increasingly higher demands on system off-axis rejection capabilities, thus requiring very low scatter surfaces to be used for critical elements of the optical train. Particulate contamination, which is deposited on mirrors, filters and/or baffles during fabrication, sensor testing, storage, launch, deployment or operation, significantly raises the quantity of scattered energy falling on the focal plane, thereby causing unacceptable signal to noise performance. Contamination due to molecular outgassing, corrosion or pitting causes greater scatter internal to the sensor's optics. When deposited on the sensor's external surfaces, molecular contamination causes a degradation of the thermal control properties. The latter can affect sensor performance indirectly by lowering detector efficiency or by causing electronic malfunctions due to increased operating temperatures. Internal molecular contamination can also be so severe as to affect system throughput by decreasing the efficiency of key optics by severe scattering losses, bulk absorption or thin film interference.
The only known contamination detector is a photometer manufactured by Saab Aktienbolaget. It is useful for detecting only particulate matter. That device uses black glass witness samples which, once exposed, are placed in the photometer. The photometer comprises a low power microscope with a light meter. Because it requires a black glass witness sample, it requires that the sample plate be removed from the environment where the sample was obtained, and then read out at a remote location to record the particulate matter level. It is also incapable of detecting such surface discontinuities as craters or molecular deposits, which wet a surface of an optical element.